Hypertext Transfer Protocol Version 3 (HTTP/3)

The QUIC transport protocol has several features that are desirable in a transport for HTTP, such as stream multiplexing, per-stream flow control, and low-latency connection establishment. This document describes a mapping of HTTP semantics over QUIC. This document also identifies HTTP/2 features that are subsumed by QUIC, and describes how HTTP/2 extensions can be ported to HTTP/3.¶

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.¶

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.¶

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

HTTP semantics are used for a broad range of services on the Internet. These semantics have commonly been used with two different TCP mappings, HTTP/1.1 and HTTP/2. HTTP/2 introduced a framing and multiplexing layer to improve latency without modifying the transport layer. However, TCP’s lack of visibility into parallel requests in both mappings limited the possible performance gains.¶

The QUIC transport protocol incorporates stream multiplexing and per-stream flow control, similar to that provided by the HTTP/2 framing layer. By providing reliability at the stream level and congestion control across the entire connection, it has the capability to improve the performance of HTTP compared to a TCP mapping. QUIC also incorporates TLS 1.3 at the transport layer, offering comparable security to running TLS over TCP, but with improved connection setup latency (unless TCP Fast Open [RFC7413]} is used).¶

This document defines a mapping of HTTP semantics over the QUIC transport protocol, drawing heavily on the design of HTTP/2. This document identifies HTTP/2 features that are subsumed by QUIC, and describes how the other features can be implemented atop QUIC.¶

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Field definitions are given in Augmented Backus-Naur Form (ABNF), as defined in [RFC5234].¶

Protocol elements called “frames” exist in both this document and [QUIC-TRANSPORT]. Where frames from [QUIC-TRANSPORT] are referenced, the frame name will be prefaced with “QUIC.” For example, “QUIC CONNECTION_CLOSE frames.” References without this preface refer to frames defined in Section 4.2.¶

RFC Editor’s Note: Please remove this section prior to publication of a final version of this document.

HTTP/3 uses the token “h3” to identify itself in ALPN and Alt-Svc. Only implementations of the final, published RFC can identify themselves as “h3”. Until such an RFC exists, implementations MUST NOT identify themselves using this string.¶

Implementations of draft versions of the protocol MUST add the string “-“ and the corresponding draft number to the identifier. For example, draft-ietf-quic-http-01 is identified using the string “h3-01”.¶

Non-compatible experiments that are based on these draft versions MUST append the string “-“ and an experiment name to the identifier. For example, an experimental implementation based on draft-ietf-quic-http-09 which reserves an extra stream for unsolicited transmission of 1980s pop music might identify itself as “h3-09-rickroll”. Note that any label MUST conform to the “token” syntax defined in Section 3.2.6 of [RFC7230]. Experimenters are encouraged to coordinate their experiments on the quic@ietf.org mailing list.¶

An HTTP origin advertises the availability of an equivalent HTTP/3 endpoint via the Alt-Svc HTTP response header field or the HTTP/2 ALTSVC frame ([ALTSVC]), using the ALPN token defined in Section 2.3.¶

For example, an origin could indicate in an HTTP response that HTTP/3 was available on UDP port 50781 at the same hostname by including the following header field:¶

Alt-Svc: h3=":50781"

On receipt of an Alt-Svc record indicating HTTP/3 support, a client MAY attempt to establish a QUIC connection to the indicated host and port and, if successful, send HTTP requests using the mapping described in this document.¶

Connectivity problems (e.g. firewall blocking UDP) can result in QUIC connection establishment failure, in which case the client SHOULD continue using the existing connection or try another alternative endpoint offered by the origin.¶

Servers MAY serve HTTP/3 on any UDP port, since an alternative always includes an explicit port.¶

This document defines the “quic” parameter for Alt-Svc, which MAY be used to provide version-negotiation hints to HTTP/3 clients. QUIC versions are four-byte sequences with no additional constraints on format. Leading zeros SHOULD be omitted for brevity.¶

Where multiple versions are listed, the order of the values reflects the server’s preference (with the first value being the most preferred version). Reserved versions MAY be listed, but unreserved versions which are not supported by the alternative SHOULD NOT be present in the list. Origins MAY omit supported versions for any reason.¶

Clients MUST ignore any included versions which they do not support. The “quic” parameter MUST NOT occur more than once; clients SHOULD process only the first occurrence.¶

For example, suppose a server supported both version 0x00000001 and the version rendered in ASCII as “Q034”. If it also opted to include the reserved version (from Section 15 of [QUIC-TRANSPORT]) 0x1abadaba, it could specify the following header field:¶

Alt-Svc: h3=":49288";quic="1,1abadaba,51303334"

A client acting on this header field would drop the reserved version (not supported), then attempt to connect to the alternative using the first version in the list which it does support, if any.¶

HTTP/3 relies on QUIC as the underlying transport. The QUIC version being used MUST use TLS version 1.3 or greater as its handshake protocol. HTTP/3 clients MUST indicate the target domain name during the TLS handshake. This may be done using the Server Name Indication (SNI) [RFC6066] extension to TLS or using some other mechanism.¶

QUIC connections are established as described in [QUIC-TRANSPORT]. During connection establishment, HTTP/3 support is indicated by selecting the ALPN token “h3” in the TLS handshake. Support for other application-layer protocols MAY be offered in the same handshake.¶

While connection-level options pertaining to the core QUIC protocol are set in the initial crypto handshake, HTTP/3-specific settings are conveyed in the SETTINGS frame. After the QUIC connection is established, a SETTINGS frame (Section 4.2.5) MUST be sent by each endpoint as the initial frame of their respective HTTP control stream (see Section 3.2.1).¶

Once a connection exists to a server endpoint, this connection MAY be reused for requests with multiple different URI authority components. The client MAY send any requests for which the client considers the server authoritative.¶

An authoritative HTTP/3 endpoint is typically discovered because the client has received an Alt-Svc record from the request’s origin which nominates the endpoint as a valid HTTP Alternative Service for that origin. As required by [RFC7838], clients MUST check that the nominated server can present a valid certificate for the origin before considering it authoritative. Clients MUST NOT assume that an HTTP/3 endpoint is authoritative for other origins without an explicit signal.¶

A server that does not wish clients to reuse connections for a particular origin can indicate that it is not authoritative for a request by sending a 421 (Misdirected Request) status code in response to the request (see Section 9.1.2 of [RFC7540]).¶

The considerations discussed in Section 9.1 of [RFC7540] also apply to the management of HTTP/3 connections.¶

A QUIC stream provides reliable in-order delivery of bytes, but makes no guarantees about order of delivery with regard to bytes on other streams. On the wire, data is framed into QUIC STREAM frames, but this framing is invisible to the HTTP framing layer. The transport layer buffers and orders received QUIC STREAM frames, exposing the data contained within as a reliable byte stream to the application. Although QUIC permits out-of-order delivery within a stream HTTP/3 does not make use of this feature.¶

QUIC streams can be either unidirectional, carrying data only from initiator to receiver, or bidirectional. Streams can be initiated by either the client or the server. For more detail on QUIC streams, see Section 2 of [QUIC-TRANSPORT].¶

When HTTP headers and data are sent over QUIC, the QUIC layer handles most of the stream management. HTTP does not need to do any separate multiplexing when using QUIC - data sent over a QUIC stream always maps to a particular HTTP transaction or connection context.¶

All client-initiated bidirectional streams are used for HTTP requests and responses. A bidirectional stream ensures that the response can be readily correlated with the request. This means that the client’s first request occurs on QUIC stream 0, with subsequent requests on stream 4, 8, and so on. In order to permit these streams to open, an HTTP/3 client SHOULD send non-zero values for the QUIC transport parameters initial_max_stream_data_bidi_local. An HTTP/3 server SHOULD send non-zero values for the QUIC transport parameters initial_max_stream_data_bidi_remote and initial_max_bidi_streams. It is recommended that initial_max_bidi_streams be no smaller than 100, so as to not unnecessarily limit parallelism.¶

These streams carry frames related to the request/response (see Section 5.1). When a stream terminates cleanly, if the last frame on the stream was truncated, this MUST be treated as a connection error (see HTTP_MALFORMED_FRAME in Section 8.1). Streams which terminate abruptly may be reset at any point in the frame.¶

HTTP/3 does not use server-initiated bidirectional streams; clients MUST omit or specify a value of zero for the QUIC transport parameter initial_max_bidi_streams.¶

Unidirectional streams, in either direction, are used for a range of purposes. The purpose is indicated by a stream type, which is sent as a single byte header at the start of the stream. The format and structure of data that follows this header is determined by the stream type.¶

0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|Stream Type (8)|
+-+-+-+-+-+-+-+-+

Figure 1: Unidirectional Stream Header

Some stream types are reserved (Section 3.2.3). Two stream types are defined in this document: control streams (Section 3.2.1) and push streams (Section 3.2.2). Other stream types can be defined by extensions to HTTP/3; see Section 7 for more details.¶

Both clients and servers SHOULD send a value of three or greater for the QUIC transport parameter initial_max_uni_streams.¶

If the stream header indicates a stream type which is not supported by the recipient, the remainder of the stream cannot be consumed as the semantics are unknown. Recipients of unknown stream types MAY trigger a QUIC STOP_SENDING frame with an error code of HTTP_UNKNOWN_STREAM_TYPE, but MUST NOT consider such streams to be an error of any kind.¶

Implementations MAY send stream types before knowing whether the peer supports them. However, stream types which could modify the state or semantics of existing protocol components, including QPACK or other extensions, MUST NOT be sent until the peer is known to support them.¶

A sender can close or reset a unidirectional stream unless otherwise specified. A receiver MUST tolerate unidirectional streams being closed or reset prior to the reception of the unidirectional stream header.¶

A control stream is indicated by a stream type of 0x43 (ASCII ‘C’). Data on this stream consists of HTTP/3 frames, as defined in Section 4.2.¶

Each side MUST initiate a single control stream at the beginning of the connection and send its SETTINGS frame as the first frame on this stream. If the first frame of the control stream is any other frame type, this MUST be treated as a connection error of type HTTP_MISSING_SETTINGS. Only one control stream per peer is permitted; receipt of a second stream which claims to be a control stream MUST be treated as a connection error of type HTTP_WRONG_STREAM_COUNT. The sender MUST NOT close the control stream. If the control stream is closed at any point, this MUST be treated as a connection error of type HTTP_CLOSED_CRITICAL_STREAM.¶

A pair of unidirectional streams is used rather than a single bidirectional stream. This allows either peer to send data as soon they are able. Depending on whether 0-RTT is enabled on the connection, either client or server might be able to send stream data first after the cryptographic handshake completes.¶

A push stream is indicated by a stream type of 0x50 (ASCII ‘P’), followed by the Push ID of the promise that it fulfills, encoded as a variable-length integer. The remaining data on this stream consists of HTTP/3 frames, as defined in Section 4.2, and fulfills a promised server push. Server push and Push IDs are described in Section 5.4.¶

Only servers can push; if a server receives a client-initiated push stream, this MUST be treated as a stream error of type HTTP_WRONG_STREAM_DIRECTION.¶

Each Push ID MUST only be used once in a push stream header. If a push stream header includes a Push ID that was used in another push stream header, the client MUST treat this as a connection error of type HTTP_DUPLICATE_PUSH.¶

Stream types of the format 0x1f * N are reserved to exercise the requirement that unknown types be ignored. These streams have no semantic meaning, and can be sent when application-layer padding is desired. They MAY also be sent on connections where no request data is currently being transferred. Endpoints MUST NOT consider these streams to have any meaning upon receipt.¶

The payload and length of the stream are selected in any manner the implementation chooses.¶

As discussed above, frames are carried on QUIC streams and used on control streams, request streams, and push streams. This section describes HTTP framing in QUIC. For a comparison with HTTP/2 frames, see Appendix A.2.¶

A variable-length integer that describes the length of the Frame Payload. This length does not include the Type field.

Type:

An 8-bit type for the frame.

Frame Payload:

A payload, the semantics of which are determined by the Type field.

Each frame’s payload MUST contain exactly the fields identified in its description. A frame payload that contains additional bytes after the identified fields or a frame payload that terminates before the end of the identified fields MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.¶

DATA frames MUST be associated with an HTTP request or response. If a DATA frame is received on either control stream, the recipient MUST respond with a connection error (Section 8) of type HTTP_WRONG_STREAM.¶

The PRIORITY (type=0x02) frame specifies the client-advised priority of a request, server push or placeholder.¶

A PRIORITY frame identifies an element to prioritize, and an element upon which it depends. A Prioritized ID or Dependency ID identifies a client-initiated request using the corresponding stream ID, a server push using a Push ID (see Section 4.2.6), or a placeholder using a Placeholder ID (see Section 5.3.1).¶

When a client initiates a request, a PRIORITY frame MAY be sent as the first frame of the stream, creating a dependency on an existing element. In order to ensure that prioritization is processed in a consistent order, any subsequent PRIORITY frames for that request MUST be sent on the control stream. A PRIORITY frame received after other frames on a request stream MUST be treated as a stream error of type HTTP_UNEXPECTED_FRAME.¶

If, by the time a new request stream is opened, its priority information has already been received via the control stream, the PRIORITY frame sent on the request stream MUST be ignored.¶

A two-bit field indicating the type of element being prioritized (see Table 1). When sent on a request stream, this MUST be set to 11. When sent on the control stream, this MUST NOT be set to 11.

DT (Element Dependency Type):

A two-bit field indicating the type of element being depended on (see Table 2).

Empty:

A four-bit field which MUST be zero when sent and MUST be ignored on receipt.

Prioritized Element ID:

A variable-length integer that identifies the element being prioritized. Depending on the value of Prioritized Type, this contains the Stream ID of a request stream, the Push ID of a promised resource, a Placeholder ID of a placeholder, or is absent.

Element Dependency ID:

A variable-length integer that identifies the element on which a dependency is being expressed. Depending on the value of Dependency Type, this contains the Stream ID of a request stream, the Push ID of a promised resource, the Placeholder ID of a placeholder, or is absent. For details of dependencies, see Section 5.3 and [RFC7540], Section 5.3.

Weight:

An unsigned 8-bit integer representing a priority weight for the prioritized element (see [RFC7540], Section 5.3). Add one to the value to obtain a weight between 1 and 256.

Note that unlike in [RFC7540], the root of the tree cannot be referenced using a Stream ID of 0, as in QUIC stream 0 carries a valid HTTP request. The root of the tree cannot be reprioritized. A PRIORITY frame sent on a request stream with the Prioritized Element Type set to any value other than 11 or which expresses a dependency on a request with a greater Stream ID than the current stream MUST be treated as a stream error of type HTTP_MALFORMED_FRAME. Likewise, a PRIORITY frame sent on a control stream with the Prioritized Element Type set to 11 MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.¶

When a PRIORITY frame claims to reference a request, the associated ID MUST identify a client-initiated bidirectional stream. A server MUST treat receipt of a PRIORITY frame identifying a stream of any other type as a connection error of type HTTP_MALFORMED_FRAME.¶

A PRIORITY frame that references a non-existent Push ID, a Placeholder ID greater than the server’s limit, or a Stream ID the client is not yet permitted to open MUST be treated as an HTTP_LIMIT_EXCEEDED error.¶

A PRIORITY frame received on any stream other than a request or control stream MUST be treated as a connection error of type HTTP_WRONG_STREAM.¶

PRIORITY frames received by a client MUST be treated as a stream error of type HTTP_UNEXPECTED_FRAME.¶

The CANCEL_PUSH frame (type=0x3) is used to request cancellation of a server push prior to the push stream being received. The CANCEL_PUSH frame identifies a server push by Push ID (see Section 4.2.6), encoded as a variable-length integer.¶

When a server receives this frame, it aborts sending the response for the identified server push. If the server has not yet started to send the server push, it can use the receipt of a CANCEL_PUSH frame to avoid opening a push stream. If the push stream has been opened by the server, the server SHOULD send a QUIC RESET_STREAM frame on that stream and cease transmission of the response.¶

A server can send the CANCEL_PUSH frame to indicate that it will not be fulfilling a promise prior to creation of a push stream. Once the push stream has been created, sending CANCEL_PUSH has no effect on the state of the push stream. A QUIC RESET_STREAM frame SHOULD be used instead to abort transmission of the server push response.¶

A CANCEL_PUSH frame is sent on the control stream. Receiving a CANCEL_PUSH frame on a stream other than the control stream MUST be treated as a stream error of type HTTP_WRONG_STREAM.¶

The SETTINGS frame (type=0x4) conveys configuration parameters that affect how endpoints communicate, such as preferences and constraints on peer behavior. Individually, a SETTINGS parameter can also be referred to as a “setting”; the identifier and value of each setting parameter can be referred to as a “setting identifier” and a “setting value”.¶

SETTINGS frames always apply to a connection, never a single stream. A SETTINGS frame MUST be sent as the first frame of each control stream (see Section 3.2.1) by each peer, and MUST NOT be sent subsequently or on any other stream. If an endpoint receives a SETTINGS frame on a different stream, the endpoint MUST respond with a connection error of type HTTP_WRONG_STREAM. If an endpoint receives a second SETTINGS frame, the endpoint MUST respond with a connection error of type HTTP_UNEXPECTED_FRAME.¶

SETTINGS parameters are not negotiated; they describe characteristics of the sending peer, which can be used by the receiving peer. However, a negotiation can be implied by the use of SETTINGS - each peer uses SETTINGS to advertise a set of supported values. The definition of the setting would describe how each peer combines the two sets to conclude which choice will be used. SETTINGS does not provide a mechanism to identify when the choice takes effect.¶

Different values for the same parameter can be advertised by each peer. For example, a client might be willing to consume a very large response header, while servers are more cautious about request size.¶

Parameters MUST NOT occur more than once in the SETTINGS frame. A receiver MAY treat the presence of the same parameter more than once as a connection error of type HTTP_MALFORMED_FRAME.¶

The payload of a SETTINGS frame consists of zero or more parameters, each consisting of an unsigned 16-bit setting identifier and a value which uses the QUIC variable-length integer encoding.¶

The default value is 0. However, this value SHOULD be set to a non-zero value by servers. See Section 5.3.1 for usage.

Setting identifiers of the format 0x?a?a are reserved to exercise the requirement that unknown identifiers be ignored. Such settings have no defined meaning. Endpoints SHOULD include at least one such setting in their SETTINGS frame. Endpoints MUST NOT consider such settings to have any meaning upon receipt.¶

Because the setting has no defined meaning, the value of the setting can be any value the implementation selects.¶

Additional settings can be defined by extensions to HTTP/3; see Section 7 for more details.¶

An HTTP implementation MUST NOT send frames or requests which would be invalid based on its current understanding of the peer’s settings. All settings begin at an initial value, and are updated upon receipt of a SETTINGS frame. For servers, the initial value of each client setting is the default value.¶

For clients using a 1-RTT QUIC connection, the initial value of each server setting is the default value. When a 0-RTT QUIC connection is being used, the initial value of each server setting is the value used in the previous session. Clients MUST store the settings the server provided in the session being resumed and MUST comply with stored settings until the current server settings are received.¶

A server can remember the settings that it advertised, or store an integrity-protected copy of the values in the ticket and recover the information when accepting 0-RTT data. A server uses the HTTP/3 settings values in determining whether to accept 0-RTT data.¶

A server MAY accept 0-RTT and subsequently provide different settings in its SETTINGS frame. If 0-RTT data is accepted by the server, its SETTINGS frame MUST NOT reduce any limits or alter any values that might be violated by the client with its 0-RTT data.¶

QPACK-compressed request header fields for the promised response. See [QPACK] for more details.

A server MUST NOT use a Push ID that is larger than the client has provided in a MAX_PUSH_ID frame (Section 4.2.8) and MUST NOT use the same Push ID in multiple PUSH_PROMISE frames. A client MUST treat receipt of a PUSH_PROMISE that contains a larger Push ID than the client has advertised or a Push ID which has already been promised as a connection error of type HTTP_MALFORMED_FRAME.¶

If a PUSH_PROMISE frame is received on either control stream, the recipient MUST respond with a connection error (Section 8) of type HTTP_WRONG_STREAM.¶

See Section 5.4 for a description of the overall server push mechanism.¶

The GOAWAY frame (type=0x7) is used to initiate graceful shutdown of a connection by a server. GOAWAY allows a server to stop accepting new requests while still finishing processing of previously received requests. This enables administrative actions, like server maintenance. GOAWAY by itself does not close a connection.¶

The GOAWAY frame is always sent on the control stream. It carries a QUIC Stream ID for a client-initiated bidirectional stream encoded as a variable-length integer. A client MUST treat receipt of a GOAWAY frame containing a Stream ID of any other type as a connection error of type HTTP_WRONG_STREAM.¶

Clients do not need to send GOAWAY to initiate a graceful shutdown; they simply stop making new requests. A server MUST treat receipt of a GOAWAY frame on any stream as a connection error (Section 8) of type HTTP_UNEXPECTED_FRAME.¶

The GOAWAY frame applies to the connection, not a specific stream. A client MUST treat a GOAWAY frame on a stream other than the control stream as a connection error (Section 8) of type HTTP_UNEXPECTED_FRAME.¶

The MAX_PUSH_ID frame (type=0xD) is used by clients to control the number of server pushes that the server can initiate. This sets the maximum value for a Push ID that the server can use in a PUSH_PROMISE frame. Consequently, this also limits the number of push streams that the server can initiate in addition to the limit set by the QUIC MAX_STREAM_ID frame.¶

The MAX_PUSH_ID frame is always sent on the control stream. Receipt of a MAX_PUSH_ID frame on any other stream MUST be treated as a connection error of type HTTP_WRONG_STREAM.¶

A server MUST NOT send a MAX_PUSH_ID frame. A client MUST treat the receipt of a MAX_PUSH_ID frame as a connection error of type HTTP_UNEXPECTED_FRAME.¶

The maximum Push ID is unset when a connection is created, meaning that a server cannot push until it receives a MAX_PUSH_ID frame. A client that wishes to manage the number of promised server pushes can increase the maximum Push ID by sending MAX_PUSH_ID frames as the server fulfills or cancels server pushes.¶

The MAX_PUSH_ID frame carries a single variable-length integer that identifies the maximum value for a Push ID that the server can use (see Section 4.2.6). A MAX_PUSH_ID frame cannot reduce the maximum Push ID; receipt of a MAX_PUSH_ID that contains a smaller value than previously received MUST be treated as a connection error of type HTTP_MALFORMED_FRAME.¶

The DUPLICATE_PUSH frame carries a single variable-length integer that identifies the Push ID of a resource that the server has previously promised (see Section 4.2.6).¶

This frame allows the server to use the same server push in response to multiple concurrent requests. Referencing the same server push ensures that a promise can be made in relation to every response in which server push might be needed without duplicating request headers or pushed responses.¶

Allowing duplicate references to the same Push ID is primarily to reduce duplication caused by concurrent requests. A server SHOULD avoid reusing a Push ID over a long period. Clients are likely to consume server push responses and not retain them for reuse over time. Clients that see a DUPLICATE_PUSH that uses a Push ID that they have since consumed and discarded are forced to ignore the DUPLICATE_PUSH.¶

Frame types of the format 0xb + (0x1f * N) are reserved to exercise the requirement that unknown types be ignored (Section 7). These frames have no semantic value, and can be sent when application-layer padding is desired. They MAY also be sent on connections where no request data is currently being transferred. Endpoints MUST NOT consider these frames to have any meaning upon receipt.¶

The payload and length of the frames are selected in any manner the implementation chooses.¶

A client sends an HTTP request on a client-initiated bidirectional QUIC stream. A client MUST send only a single request on a given stream. A server sends one or more HTTP responses on the same stream as the request, as detailed below.¶

A server MAY interleave one or more PUSH_PROMISE frames (see Section 4.2.6) with the frames of a response message. These PUSH_PROMISE frames are not part of the response; see Section 5.4 for more details.¶

Trailing header fields are carried in an additional HEADERS frame following the body. Senders MUST send only one HEADERS frame in the trailers section; receivers MUST discard any subsequent HEADERS frames.¶

A response MAY consist of multiple messages when and only when one or more informational responses (1xx, see [RFC7231], Section 6.2) precede a final response to the same request. Non-final responses do not contain a payload body or trailers.¶

An HTTP request/response exchange fully consumes a bidirectional QUIC stream. After sending a request, a client MUST close the stream for sending. Unless using the CONNECT method (see Section 5.2), clients MUST NOT make stream closure dependent on receiving a response to their request. After sending a final response, the server MUST close the stream for sending. At this point, the QUIC stream is fully closed.¶

When a stream is closed, this indicates the end of an HTTP message. Because some messages are large or unbounded, endpoints SHOULD begin processing partial HTTP messages once enough of the message has been received to make progress. If a client stream terminates without enough of the HTTP message to provide a complete response, the server SHOULD abort its response with the error code HTTP_INCOMPLETE_REQUEST.¶

A server can send a complete response prior to the client sending an entire request if the response does not depend on any portion of the request that has not been sent and received. When this is true, a server MAY request that the client abort transmission of a request without error by triggering a QUIC STOP_SENDING frame with error code HTTP_EARLY_RESPONSE, sending a complete response, and cleanly closing its stream. Clients MUST NOT discard complete responses as a result of having their request terminated abruptly, though clients can always discard responses at their discretion for other reasons.¶

HTTP message headers carry information as a series of key-value pairs, called header fields. For a listing of registered HTTP header fields, see the “Message Header Field” registry maintained at https://www.iana.org/assignments/message-headers.¶

Just as in previous versions of HTTP, header field names are strings of ASCII characters that are compared in a case-insensitive fashion. Properties of HTTP header field names and values are discussed in more detail in Section 3.2 of [RFC7230], though the wire rendering in HTTP/3 differs. As in HTTP/2, header field names MUST be converted to lowercase prior to their encoding. A request or response containing uppercase header field names MUST be treated as malformed.¶

As in HTTP/2, HTTP/3 uses special pseudo-header fields beginning with the ‘:’ character (ASCII 0x3a) to convey the target URI, the method of the request, and the status code for the response. These pseudo-header fields are defined in Section 8.1.2.3 and 8.1.2.4 of [RFC7540]. Pseudo-header fields are not HTTP header fields. Endpoints MUST NOT generate pseudo-header fields other than those defined in [RFC7540]. The restrictions on the use of pseudo-header fields in Section 8.1.2.1 of [RFC7540] also apply to HTTP/3.¶

HTTP/3 uses QPACK header compression as described in [QPACK], a variation of HPACK which allows the flexibility to avoid header-compression-induced head-of-line blocking. See that document for additional details.¶

An HTTP/3 implementation MAY impose a limit on the maximum size of the header it will accept on an individual HTTP message; encountering a larger message header SHOULD be treated as a stream error of type HTTP_EXCESSIVE_LOAD. If an implementation wishes to advise its peer of this limit, it can be conveyed as a number of bytes in the SETTINGS_MAX_HEADER_LIST_SIZE parameter. The size of a header list is calculated based on the uncompressed size of header fields, including the length of the name and value in bytes plus an overhead of 32 bytes for each header field.¶

Clients can cancel requests by aborting the stream (QUIC RESET_STREAM and/or STOP_SENDING frames, as appropriate) with an error code of HTTP_REQUEST_CANCELLED (Section 8.1). When the client cancels a response, it indicates that this response is no longer of interest. Implementations SHOULD cancel requests by aborting both directions of a stream.¶

When the server rejects a request without performing any application processing, it SHOULD abort its response stream with the error code HTTP_REQUEST_REJECTED. In this context, “processed” means that some data from the stream was passed to some higher layer of software that might have taken some action as a result. The client can treat requests rejected by the server as though they had never been sent at all, thereby allowing them to be retried later on a new connection. Servers MUST NOT use the HTTP_REQUEST_REJECTED error code for requests which were partially or fully processed. When a client sends a STOP_SENDING with HTTP_REQUEST_CANCELLED, a server MAY indicate the error code HTTP_REQUEST_REJECTED in the corresponding RESET_STREAM if no processing was performed. Clients MUST NOT reset streams with the HTTP_REQUEST_REJECTED error code except in response to a QUIC STOP_SENDING frame.¶

If a stream is cancelled after receiving a complete response, the client MAY ignore the cancellation and use the response. However, if a stream is cancelled after receiving a partial response, the response SHOULD NOT be used. Automatically retrying such requests is not possible, unless this is otherwise permitted (e.g., idempotent actions like GET, PUT, or DELETE).¶

The pseudo-method CONNECT ([RFC7231], Section 4.3.6) is primarily used with HTTP proxies to establish a TLS session with an origin server for the purposes of interacting with “https” resources. In HTTP/1.x, CONNECT is used to convert an entire HTTP connection into a tunnel to a remote host. In HTTP/2, the CONNECT method is used to establish a tunnel over a single HTTP/2 stream to a remote host for similar purposes.¶

A CONNECT request in HTTP/3 functions in the same manner as in HTTP/2. The request MUST be formatted as described in [RFC7540], Section 8.3. A CONNECT request that does not conform to these restrictions is malformed. The request stream MUST NOT be closed at the end of the request.¶

A proxy that supports CONNECT establishes a TCP connection ([RFC0793]) to the server identified in the “:authority” pseudo-header field. Once this connection is successfully established, the proxy sends a HEADERS frame containing a 2xx series status code to the client, as defined in [RFC7231], Section 4.3.6.¶

All DATA frames on the stream correspond to data sent or received on the TCP connection. Any DATA frame sent by the client is transmitted by the proxy to the TCP server; data received from the TCP server is packaged into DATA frames by the proxy. Note that the size and number of TCP segments is not guaranteed to map predictably to the size and number of HTTP DATA or QUIC STREAM frames.¶

The TCP connection can be closed by either peer. When the client ends the request stream (that is, the receive stream at the proxy enters the “Data Recvd” state), the proxy will set the FIN bit on its connection to the TCP server. When the proxy receives a packet with the FIN bit set, it will terminate the send stream that it sends to the client. TCP connections which remain half-closed in a single direction are not invalid, but are often handled poorly by servers, so clients SHOULD NOT close a stream for sending while they still expect to receive data from the target of the CONNECT.¶

A TCP connection error is signaled with QUIC RESET_STREAM frame. A proxy treats any error in the TCP connection, which includes receiving a TCP segment with the RST bit set, as a stream error of type HTTP_CONNECT_ERROR (Section 8.1). Correspondingly, a proxy MUST send a TCP segment with the RST bit set if it detects an error with the stream or the QUIC connection.¶

HTTP/3 uses a priority scheme similar to that described in [RFC7540], Section 5.3. In this priority scheme, a given element can be designated as dependent upon another element. This information is expressed in the PRIORITY frame Section 4.2.3 which identifies the element and the dependency. The elements that can be prioritized are:¶

Requests, identified by the ID of the request stream

Pushes, identified by the Push ID of the promised resource (Section 4.2.6)

Placeholders, identified by a Placeholder ID

Taken together, the dependencies across all prioritized elements in a connection form a dependency tree. An element can depend on another element or on the root of the tree. A reference to an element which is no longer in the tree is treated as a reference to the root of the tree. The structure of the dependency tree changes as PRIORITY frames modify the dependency links between prioritized elements.¶

Due to reordering between streams, an element can also be prioritized which is not yet in the tree. Such elements are added to the tree with the requested priority.¶

When a prioritized element is first created, it has a default initial weight of 16 and a default dependency. Requests and placeholders are dependent on the root of the priority tree; pushes are dependent on the client request on which the PUSH_PROMISE frame was sent.¶

Requests may override the default intial values by including a PRIORTIY frame (see Section 4.2.3) at the beginning of the stream. These priorities can be updated by sending a PRIORITY frame on the control stream.¶

In HTTP/2, certain implementations used closed or unused streams as placeholders in describing the relative priority of requests. This created confusion as servers could not reliably identify which elements of the priority tree could be discarded safely. Clients could potentially reference closed streams long after the server had discarded state, leading to disparate views of the prioritization the client had attempted to express.¶

In HTTP/3, a number of placeholders are explicitly permitted by the server using the SETTINGS_NUM_PLACEHOLDERS setting. Because the server commits to maintaining these placeholders in the prioritization tree, clients can use them with confidence that the server will not have discarded the state. Clients MUST NOT send the SETTINGS_NUM_PLACEHOLDERS setting; receipt of this setting by a server MUST be treated as a connection error of type HTTP_WRONG_SETTING_DIRECTION.¶

Placeholders are identified by an ID between zero and one less than the number of placeholders the server has permitted.¶

Like streams, placeholders have priority information associated with them.¶

Because placeholders will be used to “root” any persistent structure of the tree which the client cares about retaining, servers can aggressively prune inactive regions from the priority tree. For prioritization purposes, a node in the tree is considered “inactive” when the corresponding stream has been closed for at least two round-trip times (using any reasonable estimate available on the server). This delay helps mitigate race conditions where the server has pruned a node the client believed was still active and used as a Stream Dependency.¶

In the example in Figure 13, P represents a Placeholder, A represents an active node, and I represents an inactive node. In the first step, the server discards two inactive branches (each a single node). In the second step, the server condenses an interior inactive node. Note that these transformations will result in no change in the resources allocated to a particular active stream.¶

Clients SHOULD assume the server is actively performing such pruning and SHOULD NOT declare a dependency on a stream it knows to have been closed.¶

HTTP/3 server push is similar to what is described in HTTP/2 [RFC7540], but uses different mechanisms.¶

Each server push is identified by a unique Push ID. This Push ID is used in a single PUSH_PROMISE frame (see Section 4.2.6) which carries the request headers, possibly included in one or more DUPLICATE_PUSH frames (see Section 4.2.9), then included with the push stream which ultimately fulfills those promises.¶

Server push is only enabled on a connection when a client sends a MAX_PUSH_ID frame (see Section 4.2.8). A server cannot use server push until it receives a MAX_PUSH_ID frame. A client sends additional MAX_PUSH_ID frames to control the number of pushes that a server can promise. A server SHOULD use Push IDs sequentially, starting at 0. A client MUST treat receipt of a push stream with a Push ID that is greater than the maximum Push ID as a connection error of type HTTP_LIMIT_EXCEEDED.¶

The header of the request message is carried by a PUSH_PROMISE frame (see Section 4.2.6) on the request stream which generated the push. This allows the server push to be associated with a client request. Ordering of a PUSH_PROMISE in relation to certain parts of the response is important (see Section 8.2.1 of [RFC7540]). Promised requests MUST conform to the requirements in Section 8.2 of [RFC7540].¶

The same server push can be associated with additional client requests using a DUPLICATE_PUSH frame (see Section 4.2.9). Ordering of a DUPLICATE_PUSH in relation to certain parts of the response is similarly important. Due to reordering, DUPLICATE_PUSH frames can arrive before the corresponding PUSH_PROMISE frame, in which case the request headers of the push would not be immediately available. Clients which receive a DUPLICATE_PUSH frame for an as-yet-unknown Push ID can either delay generating new requests for content referenced following the DUPLICATE_PUSH frame until the request headers become available, or can initiate requests for discovered resources and cancel the requests if the requested resource is already being pushed.¶

When a server later fulfills a promise, the server push response is conveyed on a push stream (see Section 3.2.2). The push stream identifies the Push ID of the promise that it fulfills, then contains a response to the promised request using the same format described for responses in Section 5.1.¶

If a promised server push is not needed by the client, the client SHOULD send a CANCEL_PUSH frame. If the push stream is already open or opens after sending the CANCEL_PUSH frame, a QUIC STOP_SENDING frame with an appropriate error code can also be used (e.g., HTTP_PUSH_REFUSED, HTTP_PUSH_ALREADY_IN_CACHE; see Section 8). This asks the server not to transfer additional data and indicates that it will be discarded upon receipt.¶

Each QUIC endpoint declares an idle timeout during the handshake. If the connection remains idle (no packets received) for longer than this duration, the peer will assume that the connection has been closed. HTTP/3 implementations will need to open a new connection for new requests if the existing connection has been idle for longer than the server’s advertised idle timeout, and SHOULD do so if approaching the idle timeout.¶

HTTP clients are expected to request that the transport keep connections open while there are responses outstanding for requests or server pushes, as described in Section 19.2 of [QUIC-TRANSPORT]. If the client is not expecting a response from the server, allowing an idle connection to time out is preferred over expending effort maintaining a connection that might not be needed. A gateway MAY maintain connections in anticipation of need rather than incur the latency cost of connection establishment to servers. Servers SHOULD NOT actively keep connections open.¶

Even when a connection is not idle, either endpoint can decide to stop using the connection and let the connection close gracefully. Since clients drive request generation, clients perform a connection shutdown by not sending additional requests on the connection; responses and pushed responses associated to previous requests will continue to completion. Servers perform the same function by communicating with clients.¶

Servers initiate the shutdown of a connection by sending a GOAWAY frame (Section 4.2.7). The GOAWAY frame indicates that client-initiated requests on lower stream IDs were or might be processed in this connection, while requests on the indicated stream ID and greater were rejected. This enables client and server to agree on which requests were accepted prior to the connection shutdown. This identifier MAY be lower than the stream limit identified by a QUIC MAX_STREAM_ID frame, and MAY be zero if no requests were processed. Servers SHOULD NOT increase the QUIC MAX_STREAM_ID limit after sending a GOAWAY frame.¶

Once GOAWAY is sent, the server MUST reject requests sent on streams with an identifier greater than or equal to the indicated last Stream ID. Clients MUST NOT send new requests on the connection after receiving GOAWAY, although requests might already be in transit. A new connection can be established for new requests.¶

If the client has sent requests on streams with a Stream ID greater than or equal to that indicated in the GOAWAY frame, those requests are considered rejected (Section 5.1.2). Clients SHOULD cancel any requests on streams above this ID. Servers MAY also reject requests on streams below the indicated ID if these requests were not processed.¶

Requests on Stream IDs less than the Stream ID in the GOAWAY frame might have been processed; their status cannot be known until they are completed successfully, reset individually, or the connection terminates.¶

Servers SHOULD send a GOAWAY frame when the closing of a connection is known in advance, even if the advance notice is small, so that the remote peer can know whether a request has been partially processed or not. For example, if an HTTP client sends a POST at the same time that a server closes a QUIC connection, the client cannot know if the server started to process that POST request if the server does not send a GOAWAY frame to indicate what streams it might have acted on.¶

A client that is unable to retry requests loses all requests that are in flight when the server closes the connection. A server MAY send multiple GOAWAY frames indicating different stream IDs, but MUST NOT increase the value they send in the last Stream ID, since clients might already have retried unprocessed requests on another connection. A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last Stream ID set to the current value of QUIC’s MAX_STREAM_ID and SHOULD NOT increase the MAX_STREAM_ID thereafter. This signals to the client that a shutdown is imminent and that initiating further requests is prohibited. After allowing time for any in-flight requests (at least one round-trip time), the server MAY send another GOAWAY frame with an updated last Stream ID. This ensures that a connection can be cleanly shut down without losing requests.¶

Once all accepted requests have been processed, the server can permit the connection to become idle, or MAY initiate an immediate closure of the connection. An endpoint that completes a graceful shutdown SHOULD use the HTTP_NO_ERROR code when closing the connection.¶

An HTTP/3 implementation can immediately close the QUIC connection at any time. This results in sending a QUIC CONNECTION_CLOSE frame to the peer; the error code in this frame indicates to the peer why the connection is being closed. See Section 8 for error codes which can be used when closing a connection.¶

Before closing the connection, a GOAWAY MAY be sent to allow the client to retry some requests. Including the GOAWAY frame in the same packet as the QUIC CONNECTION_CLOSE frame improves the chances of the frame being received by clients.¶

For various reasons, the QUIC transport could indicate to the application layer that the connection has terminated. This might be due to an explicit closure by the peer, a transport-level error, or a change in network topology which interrupts connectivity.¶

If a connection terminates without a GOAWAY frame, clients MUST assume that any request which was sent, whether in whole or in part, might have been processed.¶

HTTP/3 permits extension of the protocol. Within the limitations described in this section, protocol extensions can be used to provide additional services or alter any aspect of the protocol. Extensions are effective only within the scope of a single HTTP/3 connection.¶

This applies to the protocol elements defined in this document. This does not affect the existing options for extending HTTP, such as defining new methods, status codes, or header fields.¶

Implementations MUST ignore unknown or unsupported values in all extensible protocol elements. Implementations MUST discard frames and unidirectional streams that have unknown or unsupported types. This means that any of these extension points can be safely used by extensions without prior arrangement or negotiation.¶

Extensions that could change the semantics of existing protocol components MUST be negotiated before being used. For example, an extension that changes the layout of the HEADERS frame cannot be used until the peer has given a positive signal that this is acceptable. In this case, it could also be necessary to coordinate when the revised layout comes into effect.¶

This document doesn’t mandate a specific method for negotiating the use of an extension but notes that a setting (Section 4.2.5.1) could be used for that purpose. If both peers set a value that indicates willingness to use the extension, then the extension can be used. If a setting is used for extension negotiation, the default value MUST be defined in such a fashion that the extension is disabled if the setting is omitted.¶

QUIC allows the application to abruptly terminate (reset) individual streams or the entire connection when an error is encountered. These are referred to as “stream errors” or “connection errors” and are described in more detail in [QUIC-TRANSPORT]. An endpoint MAY choose to treat a stream error as a connection error.¶

This section describes HTTP/3-specific error codes which can be used to express the cause of a connection or stream error.¶

The security considerations of HTTP/3 should be comparable to those of HTTP/2 with TLS. Note that where HTTP/2 employs PADDING frames and Padding fields in other frames to make a connection more resistant to traffic analysis, HTTP/3 can rely on QUIC PADDING frames or employ the reserved frame and stream types discussed in Section 4.2.10 and Section 3.2.3.¶

When HTTP Alternative Services is used for discovery for HTTP/3 endpoints, the security considerations of [ALTSVC] also apply.¶

Several protocol elements contain nested length elements, typically in the form of frames with an explicit length containing variable-length integers. This could pose a security risk to an incautious implementer. An implementation MUST ensure that the length of a frame exactly matches the length of the fields it contains.¶

This document establishes a registry for HTTP/3 frame type codes. The “HTTP/3 Frame Type” registry manages an 8-bit space. The “HTTP/3 Frame Type” registry operates under either of the “IETF Review” or “IESG Approval” policies [RFC8126] for values from 0x00 up to and including 0xef, with values from 0xf0 up to and including 0xff being reserved for Experimental Use.¶

While this registry is separate from the “HTTP/2 Frame Type” registry defined in [RFC7540], it is preferable that the assignments parallel each other. If an entry is present in only one registry, every effort SHOULD be made to avoid assigning the corresponding value to an unrelated operation.¶

This document establishes a registry for HTTP/3 settings. The “HTTP/3 Settings” registry manages a 16-bit space. The “HTTP/3 Settings” registry operates under the “Expert Review” policy [RFC8126] for values in the range from 0x0000 to 0xefff, with values between and 0xf000 and 0xffff being reserved for Experimental Use. The designated experts are the same as those for the “HTTP/2 Settings” registry defined in [RFC7540].¶

While this registry is separate from the “HTTP/2 Settings” registry defined in [RFC7540], it is preferable that the assignments parallel each other. If an entry is present in only one registry, every effort SHOULD be made to avoid assigning the corresponding value to an unrelated operation.¶

Registrations for error codes are required to include a description of the error code. An expert reviewer is advised to examine new registrations for possible duplication with existing error codes. Use of existing registrations is to be encouraged, but not mandated.¶

This document establishes a registry for HTTP/3 unidirectional stream types. The “HTTP/3 Stream Type” registry manages an 8-bit space. The “HTTP/3 Stream Type” registry operates under either of the “IETF Review” or “IESG Approval” policies [RFC8126] for values from 0x00 up to and including 0xef, with values from 0xf0 up to and including 0xff being reserved for Experimental Use.¶

HTTP/3 is strongly informed by HTTP/2, and bears many similarities. This section describes the approach taken to design HTTP/3, points out important differences from HTTP/2, and describes how to map HTTP/2 extensions into HTTP/3.¶

HTTP/3 begins from the premise that similarity to HTTP/2 is preferable, but not a hard requirement. HTTP/3 departs from HTTP/2 primarily where necessary to accommodate the differences in behavior between QUIC and TCP (lack of ordering, support for streams). We intend to avoid gratuitous changes which make it difficult or impossible to build extensions with the same semantics applicable to both protocols at once.¶

HTTP/3 permits use of a larger number of streams (2^62-1) than HTTP/2. The considerations about exhaustion of stream identifier space apply, though the space is significantly larger such that it is likely that other limits in QUIC are reached first, such as the limit on the connection flow control window.¶

Many framing concepts from HTTP/2 can be elided on QUIC, because the transport deals with them. Because frames are already on a stream, they can omit the stream number. Because frames do not block multiplexing (QUIC’s multiplexing occurs below this layer), the support for variable-maximum-length packets can be removed. Because stream termination is handled by QUIC, an END_STREAM flag is not required. This permits the removal of the Flags field from the generic frame layout.¶

Frame payloads are largely drawn from [RFC7540]. However, QUIC includes many features (e.g., flow control) which are also present in HTTP/2. In these cases, the HTTP mapping does not re-implement them. As a result, several HTTP/2 frame types are not required in HTTP/3. Where an HTTP/2-defined frame is no longer used, the frame ID has been reserved in order to maximize portability between HTTP/2 and HTTP/3 implementations. However, even equivalent frames between the two mappings are not identical.¶

Many of the differences arise from the fact that HTTP/2 provides an absolute ordering between frames across all streams, while QUIC provides this guarantee on each stream only. As a result, if a frame type makes assumptions that frames from different streams will still be received in the order sent, HTTP/3 will break them.¶

For example, implicit in the HTTP/2 prioritization scheme is the notion of in-order delivery of priority changes (i.e., dependency tree mutations): since operations on the dependency tree such as reparenting a subtree are not commutative, both sender and receiver must apply them in the same order to ensure that both sides have a consistent view of the stream dependency tree. HTTP/2 specifies priority assignments in PRIORITY frames and (optionally) in HEADERS frames. To achieve in-order delivery of priority changes in HTTP/3, PRIORITY frames are sent on the control stream and exclusive prioritization has been removed.¶

Likewise, HPACK was designed with the assumption of in-order delivery. A sequence of encoded header blocks must arrive (and be decoded) at an endpoint in the same order in which they were encoded. This ensures that the dynamic state at the two endpoints remains in sync. As a result, HTTP/3 uses a modified version of HPACK, described in [QPACK].¶

Frame type definitions in HTTP/3 often use the QUIC variable-length integer encoding. In particular, Stream IDs use this encoding, which allow for a larger range of possible values than the encoding used in HTTP/2. Some frames in HTTP/3 use an identifier rather than a Stream ID (e.g. Push IDs in PRIORITY frames). Redefinition of the encoding of extension frame types might be necessary if the encoding includes a Stream ID.¶

Because the Flags field is not present in generic HTTP/3 frames, those frames which depend on the presence of flags need to allocate space for flags as part of their frame payload.¶

Other than this issue, frame type HTTP/2 extensions are typically portable to QUIC simply by replacing Stream 0 in HTTP/2 with a control stream in HTTP/3. HTTP/3 extensions will not assume ordering, but would not be harmed by ordering, and would be portable to HTTP/2 in the same manner.¶

The PUSH_PROMISE does not reference a stream; instead the push stream references the PUSH_PROMISE frame using a Push ID. See Section 4.2.6.

PING (0x6):

PING frames do not exist, since QUIC provides equivalent functionality.

GOAWAY (0x7):

GOAWAY is sent only from server to client and does not contain an error code. See Section 4.2.7.

WINDOW_UPDATE (0x8):

WINDOW_UPDATE frames do not exist, since QUIC provides flow control.

CONTINUATION (0x9):

CONTINUATION frames do not exist; instead, larger HEADERS/PUSH_PROMISE frames than HTTP/2 are permitted.

Frame types defined by extensions to HTTP/2 need to be separately registered for HTTP/3 if still applicable. The IDs of frames defined in [RFC7540] have been reserved for simplicity. See Section 10.3.¶

An important difference from HTTP/2 is that settings are sent once, at the beginning of the connection, and thereafter cannot change. This eliminates many corner cases around synchronization of changes.¶

Some transport-level options that HTTP/2 specifies via the SETTINGS frame are superseded by QUIC transport parameters in HTTP/3. The HTTP-level options that are retained in HTTP/3 have the same value as in HTTP/2.¶

In HTTP/3, setting values are variable-length integers (6, 14, 30, or 62 bits long) rather than fixed-length 32-bit fields as in HTTP/2. This will often produce a shorter encoding, but can produce a longer encoding for settings which use the full 32-bit space. Settings ported from HTTP/2 might choose to redefine the format of their settings to avoid using the 62-bit encoding.¶

Settings need to be defined separately for HTTP/2 and HTTP/3. The IDs of settings defined in [RFC7540] have been reserved for simplicity. See Section 10.4.¶

HTTP_REQUEST_REJECTED (in Section 8.1) is used to indicate that a request was not processed. Otherwise, not applicable because QUIC handles stream management. A STREAM_ID_ERROR at the QUIC layer is used for streams that are improperly opened.